The present invention relates to a secondary ion mass spectroscopic method (SIMS) and also a mass spectrometer for implementing this method. Furthermore, the present invention relates to a use of such a method or of such a mass spectrometer for examining alkali metals in a sample which comprises an insulating material or is an insulator.
In microelectronics, mobile ions, such as e.g. sodium and potassium, play a very important role. If these are present as impurities in electronic components, then they can diffuse very rapidly into the dielectric layers as a result of the applied voltages. Such impurities can then significantly impair the properties of a component. Therefore, in order to optimise the reliability and lifespan of an electronic component, a sensitive and quantitative detection method for alkali metals is required. Also the properties of glasses are greatly influenced by alkali metals. Great significance is attributed to alkali metals, such as e.g. sodium and potassium, in glass corrosion.
Secondary ion mass spectrometry (SIMS) is an established method for determining the depth distribution of elements and compounds in thin layers. The surface is thereby bombarded with primary ions of a few keV energy and the emitted secondary ions are analysed with a mass spectrometer. By scanning a focused primary ion beam, a uniform removal on a surface to be analysed can be achieved. Thus the secondary ion intensities can be determined as a function of the sputtered depth. SIMS is distinguished by particularly low detection limits in ppm into the ppb range. The thus obtained intensity profiles (intensity versus measuring time) can be calibrated correspondingly by suitable measurements of the removal depth and can be converted into depth profiles.
The secondary ion yield (defined as number of emitted secondary ions per primary ion) for a chemical element depends very greatly upon the chemical environment. This can be exploited by using primary ions which are reactive for the removal. By implanting these primary ions, the chemistry of the surface is changed such that an increased secondary ion yield is achieved. It is known that, by using oxygen primary ions (O2+), the emission of positive secondary ions is enhanced. This is generally used for sensitive detection of electropositive elements, such as metals and alkali metals. The secondary ion yield of electronegative elements is enhanced by implanting caesium. Both types of primary ions are used routinely in SIMS.
Analysis of the emitted secondary ions can be effected with various mass spectrometers. Magnetic sector field devices or quadrupole mass spectrometers are established for depth profiling. Generally, the mass spectrometer is operated here as a mass filter. The intensity of the various secondary ion masses can be determined by the corresponding change in mass filter. These types of mass spectrometers operate with continuous primary ion beams.
Furthermore, also time-of-flight mass spectrometers (ToF) are widespread in SIMS. In the case of the method termed ToF-SIMS, the surface is bombarded with a short ion pulse. The desorbed secondary ions are accelerated and their flight time is measured over a suitable flight path. From this flight time, the mass of the secondary ions can be determined. With this type of spectrometer, all secondary ion masses, which are produced by a primary ion pulse, can be determined via measurement of the flight time. Therefore a parallel recording of all masses takes place. Since however the removal rate is reduced by the pulsing of the primary ion beam by several orders of magnitude, the dual-beam method is used predominantly for depth profiling. After bombardment by the primary ion pulse, the sample surface is sputtered with a second sputter ion source during the flight time measurement. In order to increase the secondary ion yield, typically oxygen- and caesium ion sources are used here as sputter ion source. As analysis source, so-called liquid metal ion sources have been successful. Examples of these are gallium-, gold- and bismuth liquid metal ion sources.
Bombardment by electrically charged primary ions and also the emission of charged secondary particles, such as electrons, positive or negative charged secondary ions, leads to a change in the surface potential in the case of insulating samples. As a result of the effect of the electrically charged region on the desorbed secondary ions, this can lead not only to a change in the energies thereof but also to a change in the trajectories thereof. Both can result in significant transmission losses in the mass spectrometer. Furthermore, this frequently also causes a significant impairment in the mass resolution. Due to the charging of the sample surface in conjunction with the primary ion bombardment, the result can also be changes in the sample. It is known that some elements can move in the sample (electromigration) under the effect of the charge. In particular alkali metals are inclined to do this. As a consequence of the charge, the intensity- or depth distributions are often very greatly falsified.
For compensation of the charges, very frequently electron beams are used. Charging of the sample surface by the primary ion beam, in the dual-beam method also by the sputter ion beam, is generally electrically positive. The negative charging of the electrons is then intended to compensate precisely for the positive charges. This can be effected by corresponding control of the current densities of the ion- and electron beams. By using an excess of very low-energy electrons with an energy of a few eV, also automatic stabilisation of the surface potential can be achieved. The surface absorbs electrons only until the surface potential corresponds to the potential of the electron source (potential of the emitting cathode of the electron source). This method is used frequently in ToF-SIMS. Here the extraction field for the secondary ions can be switched off during the flight time measurement. Thus the low-energy electrons can reach the analysis location and compensate for the charging.
Use of these charge compensation methods allows extensively reliable measurement of the secondary ion intensities. However also electromigration frequently takes place even with a significantly reduced charging of the sample. As a result of the residual low charge, some elements can be removed from the surface and thus are no longer available for desorption by the primary ion beam. This problem is particularly pronounced during depth profiling of alkali metals in insulators, such as e.g. SiO2 and glasses. Shortly after the beginning of the depth profile, the intensity of the alkali ions collapses very greatly. Only upon reaching a conductive layer below the insulating layer is the electromigration stopped. This leads to excessive concentrations and secondary ion intensities at the interface.
Depth profiling of insulators by means of SIMS is significantly complicated by charges on the sample surface. Even using electrons for charge compensation results in electromigration of some elements, such as e.g. alkali metals. If O2+ primary ions are used to increase the positive secondary ion yield, then this electromigration is particularly pronounced and prevents measurement of the correct depth distribution.
It has been shown in practice that the electromigration can be significantly reduced by the use of caesium ion sources. However, the positive ion yield is significantly reduced. The result of this is considerable impairment in the detection limits.